Experimental research on liquid evacuation was carried out for pipelines containing vertical riser sections, using air and water as medium, gas⁃liquid flow parameters were measured by high⁃speed cameras and data acquisition instruments, the liquid evacuation process is similar to the severe slug in pipeline⁃riser system. The flow characteristics at the outlet of the riser, the pressure fluctuation at the bottom and the riser pressure drop are compared. The experimental results show that when the gas pressure is low, the gas⁃liquid mixture at the riser outlet is stratified flow and slug flow, and when the gas pressure is large, it is only stratified flow. The greater the gas pressure, the faster the movement speed of the gas front, and the shorter the evacuation time. The gas front shows a tendency to break away from the upper wall surface under higher pressure, its tip is closer to the centerline of the pipe, and the slope of the liquid layer behind the gas nose is greater. The pressure at the riser bottom undergoes three processes, which correspond to the three stages of liquid production, blowout, and liquid oscillation. The greater the gas pressure, the greater the increase in pressure at the riser bottom, and the greater the corresponding peak pressure. The riser pressure drop descends more quickly and greater under higher gas pressure.